Radboud University: Drastic decline in hoverflies in Dutch forests

The numbers of hoverflies and hoverfly species have fallen drastically in the forests of the Veluwe in recent decades. In 2021, an average of 80% fewer individuals and 44% fewer species of hoverflies were counted than forty years ago. This is the first time that hoverfly numbers have been quantified in the Netherlands. The researchers publish their findings in Insect Conservation and Diversity on 12 April.

In the 1980s, the researchers recorded how primarily rare species were disappearing. “After 2000, a number of previously common species started vanishing as well,” explains author Eelke Jongejans, an ecologist at Radboud University. After bees, hoverflies are the most important pollinators in the Netherlands and play an important role in the ecosystem. When in their larval stage, a major group of hoverfly species eat aphids, and it is precisely this group that has suffered the greatest decline in the Veluwe.

Comparable to previous results
For this study, Utrecht ecologist Aat Barendregt spent forty years counting hoverflies in a mixed deciduous and coniferous forest to the west of the village of Garderen. From 1982, he carried out no less than 254 hoverfly counts along the same three-kilometre route. “It has clearly been worth our while to make accurate counts along the same route over such a long period, but the disturbing findings weigh heavily on my mind,” says Barendregt. This is the first time that these pollinators have been counted so consistently and for such a long time at a single location.

The new results confirm the massive decline in hoverflies previously found in two German studies(verwijst naar een andere website). “In one study, entomologists counted the numbers of insects, including hoverflies, they collected in six insect traps in Germany’s open grasslands,” says Jongejans, who was also involved in that earlier study. “This new study took place in a mixed forest with very different hoverfly species. The fact that the results are roughly comparable tell us that the situation is dramatic for hoverflies across ecosystems.”

Possible causes
Because the forest and its surroundings have changed little over the years, the researchers cannot put the loss of hoverflies down to the management of the forest. Instead, the cause is probably to be found in external factors.

The decline was not equally strong in all the years of the study. The steepest decline in numbers occurred in the years 1982-1990, followed by a stabilisation in the 1990s. Since 2000, the numbers have been falling sharply again. “Acid rain and nitrogen deposition are possible explanations for the decline, as is the influx of pesticides,” says Barendregt. “Although the weather conditions during and prior to the counts cannot explain the decrease in hoverflies, this does not exclude other effects of climate change.”

Red List
Theo Zeegers of the EIS Knowledge Centre for Insects, and co-author of the article, thinks that a Red List for hoverflies needs to be established soon. “This dramatic decline in hoverflies means that we need to more accurately quantify which of the 340 Dutch hoverfly species are threatened. There is no Red List for hoverflies yet, but that would be a useful tool to this end.”

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